Hydrocarbons may be produced from wells drilled below the surface of the sea. In conventional subsea drilling, a riser contains a drilling string, also known as a drilling pipe. A drilling mud column travels through the drilling string from an offshore drilling rig and downward to the bottom of a wellbore being drilled. Drilling mud then returns with cuttings upward along the outside of the drilling string in the annulus region of the riser to the drilling rig.
To drill subsea wells in a deep ocean environment, it may be useful to employ a drilling technique known as dual gradient drilling. Dual gradient drilling is a drilling technique employing drilling mud in the drill string down to the wellbore. It employs a drilling mud return path that does not follow the annulus but instead travels by a different route. In dual gradient drilling it is possible to employ a subsea rotating device above the lower marine riser package. A subsea rotating device functions to seal the annulus between the riser and the drill string. This separates drilling mud in the annular region below the subsea rotating device from seawater or seawater equivalent fluid employed in the annular region above the subsea rotating device.
When drilling in subsea environments, it is sometimes necessary to conduct an emergency disconnect of the lower marine riser package from the blowout preventer stack located adjacent to the sea floor. Emergency disconnects may occur, for example, during severe weather, when sea conditions result in high waves, winds and or currents. Emergency disconnect during dual gradient drilling operations may trap seawater or seawater equivalent fluid within the riser. A drilling vessel with a suspended freely hanging riser during severe weather conditions presents challenges due to the substantial weight of the riser suspended from the vessel. Vessels heave up and down in heavy storms, and such a suspended riser may move vertically up and down, inducing stress upon the riser, with the potential to cause failure and loss of the riser. In such conditions, it would be desirable to minimize the weight or mass of the riser. Trapped seawater or seawater equivalent fluids in the riser adds additional weight and mass to the riser, which is undesirable.
An apparatus, system and method for reliably discharging seawater or seawater equivalent fluids in the event of riser disconnect would be highly desirable. The invention is directed to such operational challenges.